Constant wattage heaters are used for many applications such as under floor heaters, home zone heaters, mirror defoggers, water heaters, in appliances, incubators, food warmers, aquarium heaters, etc. In one example, constant wattage heating is provided along a length of the heater by providing heating zones made of heating elements that are electrically coupled in parallel. Constant wattage heaters consume electrical energy at a substantially constant rate when a constant voltage is applied to the heaters. Further, the impedance or resistance of constant wattage heaters remains substantially constant during operation. However, such heaters may have a number of issues. For example, if insulation is placed over the heaters limiting the heat lost to ambient surroundings, temperatures around the heaters can increase and may possibly cause heater degradation. Such conditions may be especially relevant when large areas are heated, such as for under floor heaters.
A control element like a thermistor, thermocouple, or bimetal switch may be employed to limit the temperature of the heater. However, control elements can add cost and complexity to a simple heater. Further, a controller configured to adjust electrical power delivered to the heater based on the control element may be required to control the heater to a desired temperature. In addition, the possibility of heater control degradation increases as the number of control elements increases. The placement of temperature sensing devices may also be important to ensure desired temperature control because localized insulation or convection may affect heater temperature. One way to provide temperature control for a heater is to position temperature sensors or switches all over the surface of the heater. However, hundreds of sensors, switches, and electric connections linking sensors to a controller may be required to cover the entire heater surface depending on the size of the heater and the extent of the temperature feedback sought.
In some heater applications, the watt density “Q” (watts/m2 or watts/ft2) of a heater may be limited because higher watt densities may result in temperatures that are higher than is desired. Further, in some applications, it may be desirable to have rapid heating so as to bring an assembly or component to some desired temperature in a relatively short period of time instead of waiting to reach some equilibrium temperature. Thus, the diversity of heating requirements between different applications may make it difficult for one heater approach to be applicable to more than a single application.
Another type of heater is a positive temperature coefficient (PTC) heater. Self regulating heaters using PTC materials were pioneered by Raychem Corporation in the 1970's. Carbon based polymer inks and carbon loaded polymers are examples of PTC materials developed for electric heaters and resettable fuses. The application of the PTC materials was severely limited because the PTC material had initial high resistance 105→106 ohms/sq. Further, the materials required very close buss bar spacing and etched buss bars because the material would degrade at high watt densities. Subsequently, other companies including DuPont developed material with lower sheet resistivity of 1500 ohms/sq, but the material would increase in resistance by only three or four times. Therefore, the material lacked properties for effectively limiting current or heating. In addition, the resistance increased gradually as material temperature increased. Thus, a large change in material temperature was required to produce a large change in the resistance of the material. More recently, in U.S. Pat. No. 5,993,698 by Frentzel et al., a new screen printable ink is disclosed that exhibits a large change in ink coating resistance with a relatively small change in ink temperature.
The inventor herein has recognized the above-mentioned disadvantages of PTC and constant wattage heaters and has developed a heater, comprising: a printed or coated PTC material providing a resistance of 20 to 1500 ohms/sq, the resistance increasing three to five orders of magnitude at a threshold temperature, the printed or coated PTC material in thermal communication with a heated area, the resistance of the PTC material responsive to a temperature of the heated area.
By constructing a heater comprising PTC material with a low resistance below a trigger temperature of the PTC material and a resistance that is three to five orders of magnitude above the trigger temperature, it may be possible to provide heaters and current limiting devices that overcome limitations of materials having higher levels of resistance. Further, a heater or current limiting device comprising PTC material and a constant wattage heating element can provide improved self regulating heater control. For example, the PTC material can provide rapid heating and current regulation while the constant wattage heater can provide uniform heating and power utilization over a large heating area. In one example, a uniform coating of PTC ink traces can be configured to link two portions of a constant wattage heating element. If the constant wattage heating element temperature increases to a threshold temperature that initiates or triggers a threshold change in resistance of the uniform coating of PTC ink, current flow through the constant wattage heating element is limited via the PTC ink trace-. Similarly, if a voltage applied to the constant wattage heater is inadvertently increased to a level where current though the PTC ink trace increases to a level heating the PTC ink to a trigger temperature, current flow may be restricted by the PTC material such that degradation of the constant wattage heater may be limited. In this way, temperature and current flow through a heater may be controlled without a complex controller and control elements so that heater reliability can be increased while system complexity may be reduced.
The present description may provide several advantages. In particular, the present description provides for heater regulation absent a controller, controller sensors, and controller actuators. Further, the present description may reduce system cost since the heaters and current control devices may be fabricated relatively simply. Further still, a large variety of heating devices having different heating properties may be constructed according to the present description.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.